Add selection of the reference stream for subtitle sync
I have a movie with embedded subtitles which are not in sync, and every subtitle gets synced with this embedded subtitle so they are also not in sync. Using ffsubsync with --reference-stream 0:a:0 so it syncs with the first audio stream gives me a properly synced subtitle.
An option to select the standard reference stream, or have an option to override it for one movie, would allow a better sync for movies with bad embedded subtitles.